In a transparent conductive film composed mainly of indium, tin-doped indium oxide (ITO) is generally used. The reason is that the concentration of carriers is increased by doping with tin, whereby a transparent conductive film improved in conductivity can be obtained.
However, an ITO film has the following disadvantages. Since a strong acid (for example, aqua regia) is required to be used for etching an ITO film, when an ITO film is used as an electrode for a TFT liquid crystal, metal wiring in underlying layers may undergo corrosion by etching. In addition, an ITO target, which is used for preparing an ITO film by sputtering, tends to blacken by reduction. Change of its properties with time has become a problem.
As a transparent conductive film which is not only more improved in etching properties than ITO films but also have conductivity and light transmittance which are equivalent to those of ITO films, and as a sputtering target which is preferable to obtain such a transparent conductive film, a target or a transparent conductive film comprising indium oxide and zinc oxide has been proposed (Patent Documents 1 and 2). However, a transparent conductive film comprising indium oxide and zinc oxide are etched quickly in a weak acid, and therefore can be etched with an etchant for a thin metal film. However, when etching a thin metal film formed on a transparent conductive film, a transparent conductive film comprising indium oxide and zinc oxide may also be etched. Therefore, a transparent conductive film comprising indium oxide and zinc oxide is not suitable when selectively etching a thin metal film on a transparent conductive film.
It is known that a transparent conductive film comprising indium oxide and zinc oxide can be used also as an oxide semiconductor due to the presence of a large amount of oxygen during film formation by the sputtering method (Patent Document 3).
Furthermore, it is reported that a transparent conductive film containing indium and a lanthanoid element is effective as an electrode for an organic EL and as a semi-transmissive/semi-reflective LCD electrode (Patent Documents 4 to 12).
However, oxides of a lanthanoid-based element have no conductivity. If these oxides are mixed in indium oxide to prepare a target, insulating particles are present in the target as they are. As a result, abnormal discharge during sputtering or blackening of the target surface occurs, resulting in disadvantages such as a lowered sputtering speed.    Patent Document 1: JP-A-H6-234565    Patent Document 2: JP-A-H7-235219    Patent Document 3: US Patent Publication No. 2005/199959    Patent Document 4: JP-A-2004-68054    Patent Document 5: JP-A-2004-119272    Patent Document 6: JP-A-2004-139780    Patent Document 7: JP-A-2004-146136    Patent Document 8: JP-A-2004-158315    Patent Document 9: JP-A-2004-240091    Patent Document 10: JP-A-2004-294630    Patent Document 11: JP-A-2004-333882    Patent Document 12: JP-A-2005-314734